The present invention is related to a stripline filter comprising at least a first stripline resonator being coupled to a second stripline resonator.
The invention is also related to a receiver using such a stripline filter.
A stripline filter according to the preamble is known from published European Patent application No. 541 397.
Such filters are especially used in transmitters and receivers for high-frequency signals, such as transmitters and receivers for GSM, PCN and DECT.
GSM (Global System for Mobile Communication) is a digital cellular mobile telephony system which utilizes high-frequency signals in the 900 MHz band.
PCN (Personal Communication Network) is a digital cellular mobile telephony system intended for small portable telephones and utilizes a frequency of 1800 MHz.
DECT (Digital European Cordless Telephone) is especially intended for cordless telephony over a relatively short distance between the wireless telephone and the dedicated base station. DECT operates as does PCN at a frequency of about 1800 MHz.
The present filters are especially used for suppressing undesired signals that have a frequency lying outside the range assigned to that particular system. This suppression is necessary, because without filtering, the receiver may easily be overloaded by strong transmitters transmitting from outside this range.
The known filter utilizes at least two mutually coupled stripline resonators. The input and output of the filter may be coupled to the resonator in different ways. Several examples of such a coupling are described in the book entitled "Microwave Filters, Impedance Matching Networks and Coupling Structure" by G. L. Matthaei, L. Young and E. M. T. Jones, published by Mc Graw-Hill Book Company 1964, pages 217-229.
In the stripline filter according to the above mentioned European patent application the only ways of varying the transfer function of the stripline filter are varying the resonance frequency of the resonators and the strength of their coupling.